[LLVMdev] Copy Instructions?

[David Greene]

There is no register-to-register copy instructions in LLVM.  I've found a bug 
in mem2reg (LLVM 2.3 but it doesn't appear to be fixed in trunk either) and a 
copy instruction seems to be the easiest way to fix it.

Consider this code coming into the RenamePass phase of mem2reg.
"*" in the phi means we haven't added that incoming value yet.

          // x.0 = ...
          // y.0 = ...
          //
          // do
          //   x = phi(x.0, *)
          //   y = phi(y.0, *)
          //   [...]
          //   store x -> y
          //   store expr -> x
          // end do
          //
          // We are updating both phis to add a value from a back edge
          // When we saw the first store we set IncomingVals[y] to x.
          // When we see the second store we need to create a
          // temporary t = x before the second store and set
          // IncomingVals[y] to t.  Otherwise y's phi will pick up the
          // new value of x (expr) rather than the old, proper one.

This bug triggers on a code we have here.  I've been unable to successfully 
bugpoint it as bugpoint croaks with a bad_alloc exception.  We're in the midst
of upgrading to 2.4 so I will re-try bugpoint then.  The above pseudocode 
represents the situation adequately.

How do I go about creating the copy t = x?  I don't want to use an add with a 
zero constant because it may not be optimized in all circustations (floating 
point, for example).

In this particular example another solution would be to reorder the phis but I 
don't think that will work in the general case.

                                                    -Dave